1. Field of the Invention
The present invention relates to a channel equalizer of a terrestrial digital television (TV) receiver using a vestigial side band (VSB) modulation.
2. Discussion of the Related Art
Generally, a terrestrial television (TV) signal, which is transmitted from a transmitting terminal by using a vestigial sideband (VSB) system, arrives to a receiving terminal after passing through a multi-path channel. However, the arriving signal is seriously distorted by an Inter Symbol Interference (ISI), so that a channel equalizer should be adopted in order to remove the interference and restore the original signal.
There is a nonlinear decision feedback equalizer as the most channel equalizer adopted in a receiver for a single carrier transmission system such as the present VSB system. Since the nonlinear decision feedback equalizer works in a time domain and uses a least mean square (LMS) with a small operation amount for updating coefficient, its convergence speed is slow, but has an advantage that the implementation is simple and easy.
If the channel distortion is serious, that is, an output of the time domain decision feedback equalizer does not remove all of the ISI or the channel has a big sudden variation during normal operation, the decision value for determining the output of the nonlinear decision feedback equalizer is very different from an original signal. In this case, the miss-determined decision value is inputted into the equalizer of a feedback portion again so that the output from the nonlinear decision feedback equalizer may be deteriorated more and more, that is, there may be occurred kind of an error propagation phenomenon. Moreover, after the error propagation phenomenon is occurred, since the equalizer shows a hysteresis symptom that cannot be re-converged and remains at the deteriorated state even though the state of the transmission channel is improved, the equalizer has an equalizing performance limit under a serious static multi-path channel.
Moreover, since the time domain equalizer such as the nonlinear decision feedback equalizer works in a manner of remaining only a main path with the largest energy among the multi-path and removing all the ISI component corresponding to the rest path, the time domain equalizer cannot be stably operated when a position of the main path since such as a time varying channel not a static channel is varied, so that a frame synchronization is frequently broken. The frame synchronization should be necessary in the course of a channel decoding performed after the equalizer, and if the synchronization is broken, the channel decoding becomes impossible.
In a case of a transmission channel for the terrestrial TV, the time difference causes by the multi-path often becomes more than several tens of ps due to the characteristics of very sensitively responding to effect from the configuration of the ground and building on the ground. In a time domain equalizer working at a symbol rate (10.76 MHz), in order to restore the original signal from multi-path signal entering with the great time difference, the length of a filter of the time domain equalizer should be greater than several hundreds of tabs at minimum on the basis of a center tab. As described above, as a demand for a spread of equalizing time domain is gradually increased in the terrestrial TV receiver, the extension of the length of the time domain adapting an equalizing filter has a limit for implementation.